You get IG-88 at the start of Mission 14 (Cloud City Conflict). Like Bossk, he can Neutralize Heroes on the Galactic Map.

On Land
Both of his powers (Infection and Radioactive Contamination) have poor range, work slowly, and do not affect Turrets. However, he has enough armor and firepower to let him outgun a turret.

Against Dark Troopers and stronger weapons fire, use the Radioactive Contamination -- especially at an area being defended, as the defending units will usually give up a chase and return to their positions, even if it is under the green Radioactive Contamination haze.

His blaster fire is actually a volley of shots, and strong against turrets and infantry squads. One volley can take out about 6-7 individual soldiers.

In Space
IG-88 has his own personal ship that is something in between the size of a fighter and a corvette. His Corrupt Systems power can be useful in bringing down most of the shields on a large ship or base quickly at the start of an engagement, giving your forces a head start in damage. However, he has to be at the centre of the area you target, and in a thick firefight he's a bit fragile.
He can be useful against fighters, but even then it's iffy.